My 2002 supercrew a/c blows out of the defrost vent when I reach 70 miles per hour, under that it will work ok. Anyone have this problem

 

Likely a vacuum leak. You will need to trace all the vacuum hoses to the vent controls and engine to find the one that is leaking. It could also be the vacuum vent manifold that controls the vents that is leaking. Unless it is only blowing out of the defrost and the other vents close I would forget about it since it blows cold air. I had a Z car once that would close all the vents at 80 MPH. The dealer replaced the vacuum vent selector under warranty and it never did it again.